(a) Field of the Invention
The present invention relates to a method for the fabrication of integrated circuits (ICs), and more particularly it relates to the method for the fabrication method of alignment marks of a twin-well Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET).
(b) Description of The Prior Art
In order to produce all integrated circuit, the circuit layout patterns are transferred from photomasks to the surface of the silicon wafer using a photolithography process. Each layer followings the zero layer must be carefully aligned to the previous pattern on the wafer. Alignment marks are introduced on each mask and transferred to the wafer as part of the integrated circuit pattern. Computer-controlled alignment equipment called stepper has been developed to achieve the level of sub-micron VLSI process precision (typically 0.6 .mu.m with .+-.0.15 .mu.m tolerance). Various arrangements have been used to position the alignment marks on the photomask (see U.S. Pat. No. 4,768,883 to Waldo et al., the entire disclosure of which is herein incorporated by reference, and U.S. Pat. No. 5,406,373 to Kamon, the entire disclosure of which is also herein incorporated by reference). There are two most commonly used commercial stepper alignment methods in today's VLSI industry: one is developed by ASM company which puts two alignment marks in the middle of the wafer, as shown in FIG. 1A; the other is developed by Nikon company which puts several alignment marks along the X and Y direction scribed lines, as shown in FIG. 1B. The comparisons of those two alignment methods are discussed as follows:
(1) ASM method: The advantage of this method is that it only needs to align two global photolithography alignment marks. This can save alignment operation time, thus increasing the outputs of production lines. However, it requires the fabrication of an extra zero layer mask for every single product to produce global alignment marks, that increases the manufacturing cost.
(2) Nikon method: The advantage of this method is that there are several alignment marks for each die. This can improve the alignment accuracy. On the other hand, it measures ten field positions to take the average of them to achieve better accuracy. This takes longer time for alignment, and thus reduces the outputs of the production. Furthermore, the first layer must be subject to an etching step, otherwise there are no alignment marks for succeeding layers to align.
Because the alignment methods of the ASM stepper and the Nikon stepper are different, one particular twin-well MOS product cannot use both ASM steppers and Nikon steppers for mass-production. If we want to use the ASM stepper alignment method in a Nikon stepper, it would need an extra photomask for zero layer alignment. Therefore, it will not only increase the number of manufacturing steps but also increase the production cost.